Mozilla dramatically slowed the update pace of Firefox 9, the browser it shipped late last month.

The company also said it may repeat the slow-down in the future.

Firefox 9, which Mozilla released Dec. 20 , has yet to be completely “unthrottled,” or offered as an update to all users, according to notes from a company meeting last week.

Like other software vendors, including Microsoft and Apple, Mozilla can offer upgrades to a fraction of its users rather than to everyone at once. The practice is designed to ensure that download servers aren’t overwhelmed, and to prevent bugs—if there are any in the update—from reaching all users.

Firefox 8, the edition that launched Nov. 8, 2011, accounted for 40 percent of all versions of Mozilla’s browser five days after its release, and broke the 50 percent mark 18 days after it shipped, according to usage statistics from Irish metrics firm StatCounter. Meanwhile, Firefox 9 accounted for just 7 percent of all editions of Firefox five days after its debut and required 24 days to reach 50 percent.

The slow-down was related to several bugs in Firefox 9 that Mozilla developers have investigated, including one mentioned last week in a status meeting that blocks some users running Mac OS X 10.6, aka Snow Leopard, from playing Netflix content.

While Mozilla issued Firefox 9.0.1 a day after shipping Firefox 9—the former removed code that caused some Mac, Linux and Windows browsers to crash—it has not released a version 9.0.2 to patch any of the bugs that prompted it to throttle the upgrade process.

Firefox 9’s slow upgrade appears to conflict with Mozilla’s rapid-release schedule that ships a new version every six weeks. Mozilla, which debuted the speedier pace last summer, has touted it as a way to get new features into users’ hands faster, and security updates to them on a regular timetable.

Mozilla said it may use a slower-upgrade process for future editions.

In notes for a meeting held Jan. 3, the company said, “Probably going to do something similar for releases in the future as well.”

Mozilla did not answer questions about how the throttling seemed at odds with its rapid-release tempo—and with plans to offer a silent updating mechanism later this year—but a spokeswoman pointed out a line the company added to the release notes for Firefox 9. “We manage and monitor the rate of automatic updates to ensure a smooth release. Users can always manually check for updates from the About Firefox window to see whether they are up to date,” the release notes stated.

Firefox 10 is slated to ship two weeks from Tuesday, on Jan. 31.

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